This invention relates to synchronization of digital signals and, more particularly, to the resynchronization of pulse coded signals at an interface between two digital systems that operate according to separate time bases which are not synchronized to each other.
Digital signal operations, whether they are in a digital signal transmission system, a coder, a decoder or some kind of signal processing arrangement, require some type of predetermined time base that periodically produces timing information or signals upon which systematic functions are based to provide orderly operations. Timing signal operations or synchronization is provided by using all or selectable signals from a time base signal produced by a local oscillator or local clock. Although such oscillators are designed to produce a stable time base signal of a predetermined constant frequency, all oscillators are subject to a finite inaccuracy in frequency in addition to some value of time-varying frequency drift. Accordingly, two identically designed oscillators with the same specifications will most likely produce signals that are slightly different in frequency and have a time-varying frequency difference.
When a plurality of digital systems are connected together to communicate with one another, compatible operation requires overall synchronism of systematic functions. To provide compatible operation, the local oscillators of digital systems are generally synchronized or phase locked together to prevent the time-varying frequency drift among the various local oscillators. If the frequency drift is not eliminated, data may be lost or erroneously processed.
When various portions of an overall digital communication system are spread out to serve a geographic area, reliable synchronization of local oscillators at different locations in the system may require additional communication links to a master clock which increases the cost of these systems. As the distances are increased to cover larger areas, differences in propagation delay times of various signal paths including those used for synchronization purposes are sufficient to produce synchronization problems. Furthermore, failure of the master clock or any of the communication links used for synchronization are additional sources of system failure.
One conventional solution of providing an interface between independently synchronized digital systems is to convert the digital information of one system to analog and then reconvert the analog signal back to a digital signal that is retimed according to the time base of another system. Another technique is to convert the pulse code modulated (PCM) signal to a delta modulated signal and then reconvert back to a PCM signal in accordance with a second time base. Both arrangements utilize signal conversion which is undesirable in terms of the equipment needed to implement the conversion and amplitude approximations inherent to signal conversion techniques which degrade the quality of the information content of the signal undergoing conversion. A further drawback of the latter technique is that if the digital signals of the two systems are not encoded using the same sampling rate, it provides the rate change by simply repeating or deleting digital signals which also degrades the quality of information.
It is an object of the present invention to provide a flexible interface between cooperating but autonomously synchronized digital systems using a minimum amount of signal conversions that automatically compensates for frequency difference and drift caused by either the temporary or permanent absence of synchronization thereby improving overall system performance and reliability.